1. Field of the Invention
The present invention relates to a device for tensioning copy masters with tensioning means that include a clamp for clamping the copy masters between a moveable press on member and a counter bearing. Such device is used in particular for scanning or imaging copy masters in an apparatus which copies the copy masters on copy material, so-called photo printers. The photo printers have an objective which images the copy masters on the copy material. The objective must have a large depth of field and a high image resolution. To satisfy the stringent requirements on depth of field and image resolution, the copy masters have to be in a precise flat position during the copy process. The copy masters are therefore placed under tension for the copying operation.
2. Description of the Related Art
U.S. Pat. No. 5,028,956 describes a device for placing copy masters under tension when the copy masters are copied on copy material. This device produces a very flat film surface by placing the copy masters under tension in a transverse direction perpendicular to the transport direction of the copy masters. To tension the copy masters in the transverse direction, this device has two clamping means which guides the copy masters by clamping the masters on two opposite sides to a platen. The two clamping means are connected to each other by a spring. A force which is applied to the spring perpendicular to the plane of the copy masters, moves the two clamping means towards the copy masters, wherein the clamping means then securely clamp the copy masters on the platen. Additional pressure applied to the spring then moves one of the clamping means outwardly in the transverse direction and away from the other clamping means. A cylinder which makes contact with the copy masters, is attached to the lower end of the clamping means which can move outwardly in the transverse direction. Since the copy masters are clamped to the platen by the respective clamping means which cannot move in the transverse direction, the copy masters are placed under tension in the transverse direction when the other clamping means moves outwardly. The prior art device can only operate if the friction between the clamping means which is movable in the transverse direction, and the copy masters is greater than the friction between the copy masters and the platen guiding the copy masters.
In order to obtain such a friction ratio and to ensure that the prior art device for tensioning the copy masters in the transverse direction operates reliably, an elastomer cover can be applied to the cylinder of the outwardly movable clamping means. Progressive contamination during use, however, can disadvantageously weaken the relative large frictional force of such an elastomer cover required between the cylinder and the copy masters. The tension applied to the copy masters may also cause the elastomer cover to detach over time.